Conventional antenna diversity systems typically use one receiver path for each antenna present in the system. A receiver 100 with a three-antenna diversity receiver path is shown in FIG. 1. Receiver 100 is shown as including receiver paths 120, 140 and 160. Each receiver path is shown as including a low-noise amplifier, a frequency conversion module, one or more filters, a variable gain amplifier, collectively forming an analog front end and a baseband processor. For example, as illustrated in FIG. 1, receiver path (alternatively referred to as channel) 120 is shown as including an analog front-end 125 and a baseband processor 120. Analog front end is shown as including a low-noise amplifier 102, a frequency conversion module 104, such as a mixer, one or more filters 106, 108, and a variable gain amplifier 110.
As shown in FIG. 1, the three-antenna diversity receiver 100 is shown as including three receivers that are coupled to their associated baseband processors. In each receiver path, e.g., receiver path 120, the signal enters an RF analog front end, e.g., 125, where the signal is amplified, filtered and downconverted prior to being digitized as a baseband signal. The output signals CSi, where i is an integer varying from 1 to 3 of the baseband processors 165, 175, and 185 are combined by combiner 190 in such a way as to optimize signal quality using any one of a number of conventional algorithms, such as simple switched diversity algorithm; or optimal combining algorithm according to which the signals from each diversity channel are cophased and summed; or interference cancellation algorithm in accordance with which the signals are combined in such a way as to reduce cochannel interference (CCI). As is known, CCI degrades quality of the desired signal. A full diversity receiver such as that shown in FIG. 1 enables the component signals to be individually equalized. That is, a frequency-dependent phase and amplitude can be applied across the frequency components of each diversity signal before such signals are combined by combiner 190. However, such diversity systems require full receiver and baseband signal paths for each antenna disposed in the system.
A technique for low-complexity antenna diversity is described in an article entitled “Low-Complexity Antenna Diversity Receivers for Mobile Wireless Applications,” by Zhang, C. N., Ling, C. C., International Journal on Wireless Personal Communications, pp. 65-8. The authors show the viability of combining diversity antenna signals using front-end analog circuits to achieve significant diversity gain in comparison to conventional diversity techniques that require duplicate signal paths and modems. The technique described in the article provides hardware savings by eliminating one of the modems. Furthermore, since each antenna is receiving the same desired channel, the need for duplicate local oscillator is eliminated. Likewise, channel selection filters, amplifiers and data conversion hardware, can be shared.
Patent application Ser. No. 11/874,854, filed Oct. 18, 2007, and patent application No. 60/862,193, filed Oct. 19, 2006, both entitled “Low Complexity Antenna Diversity”, and the contents of which are incorporated herein by reference in their entirety, disclose a diversity combining receiver which combines the diversity signals prior to baseband and demodulator processing, as shown in FIG. 2.
The signals from the various channels are combined using either maximum ratio combining (MRC) technique or simple cophasing technique. In a conventional MRC technique, which accounts for both the phase and the signal-to-noise ratio for each channel, the entire signal is treated as a single band. In a conventional single-band MRC, as shown in FIG. 2, the signal received by each antenna is delivered to an associated analog front end AFi component, where i is an index of the diversity channel. One advantage of the single-band MRC technique is that it achieves significant diversity gain while requiring relatively low complexity. However, when the wireless channel through which the received signals pass has frequency-selective fading (which can be described by well-known channel models such as the 6-path Typical Urban 6 (TU-6)), the conventional single-band MRC technique may not provide as large a diversity gain as a conventional diversity receiver. For example, a two-branch diversity system using single-band MRC may only provide, for example, 2.5 dB of diversity gain compared with a conventional diversity receiver which may provide, for example, 8 dB of diversity gain. A conventional diversity receiver uses two full receivers and has double the cost, power and size of a single-band MRC.